Composition and a process for electrolytic cleaning of metals



United States Patent COMPOSITION AND A PROCESS FOR ELEC- TROLYTIC CLEANING OF METALS Hugh G. Webster, Detroit, Mich, assignor to Kolene Corporation, Deroit, Mich. N Drawing. Application November 14, .1955, Serial No. 546,797

16 Claims. ll. 204-145) This invention relates to the art of treating metals and particularly to the art of cleaning work pieces of metal. My prior patent, No. 2,468,006, issued April 19, 1949, relates to the electrolytic cleaning of metal and it is to that art that this present application also relates.

The present invention contemplates the use of a molten salt bath containing an alkali caustic or hydroxide plus a compound of an alkali metal and aboron oxide, typically a borate such as borax, for the purposes of immersing a metallic work piece therein to clean the surfaces of the work pieces of sand, scale, and other surface impurities electrolytically.

The improvement of this application over prior molten salt baths is that in this case the molten salt bath is formed of an alkali metallic caustic such as sodium hydroxide plus an alkali-boron-oxygen bearing compound wherein the oxygen is available, such as sodium borate, NazBrOq, known as borax. When this bath is subjected to electrolysis, sodium in its free state appears at the cathode. The sodium combines with oxygen made available from the alkali-boron-oxygen compound to form sodium oxide which further reacts with the bath and with surface oxides of the work piece to form compounds dissolved or suspended in the bath and oxide free cleaned metal.

The alkali borate is elfective to remove oxide from the work piece both when the Work piece is cathodic and anodic, a property which results in far more rapid cleaning of the metal including removal of graphite, a surface component usually removed when the work piece is anodic.

Moreover, I have found that fluoride, still optionally used in the cleaning only when it is necessary to remove sand inclusions in the metal surface to be cleaned, cooperates with the borate for more rapid cleaning since fluoride operates to remove sand when the workpiece is either anodic or cathodic. Accordingly, when fluoride is present in the bath, the surface oxide scale of the metal to be cleaned, including sand, which may be intimately contained in the oxide scale, is removed cooperatively by both the borate and fluoride to remove the entire coating of scale and sand included therein very rapidly. For instance, using an alkali borate compound in the fused bath as a typical operation, the final electrolytic reducreduced to as little as minutes in contrast to three times as long usually necessitated by other baths, because scale is removed both at anode and cathode by periodic reversal of the current flow, and when sand is present tion usually applied when current is reversed may be,

it too is removed at both electrodes cooperatively with Y the metal oxide removed by borate.

Accordingly, the simplest bath composition of this invention will comprise to percent by Weight of caustic, typically sodium hydroxide, and at least 5 permetal chloride such as sodium chloride.

a drated form in use.

' tetraborate since it is most economical and cheap as a 2,760,927 Patented Aug. 28, 1956 it will be used in quantity ranging from about 1 to 10% of fluoride, and the borate from 1 to 1-5 percent by weight, and for purposes of economy still in the range given, the quantity of borate may be reduced so that the combined quantities of borate and fluoride are at least 5% However, larger quantities where desired, may be, but not essentially, used, and the quantities of each component may range as high as the range given.

The bath may further, but not essentially, have added thereto small quantites, such as from 1 to 15% of an alkali This chloride is usually present only for purposes merely of controlling the liquidity of the bath to a desired viscosity when molten, as well as to vary the electrical-conductivity thereof. For these specific functions the alkali fluoride also has some effect, and where the fluoride is omitted, the control of conductivity and liquidity of the bath is preferably effected entirely by the addition of chloride. Both fluoride and chloride may be present, the chloride being the cheapest and most readily available liquid and conductivity control agent. Of course, the caustic soda and borate containing bath will operate to clean metal with both the chloride and the fluoride omitted.

The passage of current through the bath effects both oxidation and reduction at the respective electrodes so that the oxidized character of the alkali borate, the ratio of alkali to boron and oxygen, and the presence or absence of water of crystallization is not critical since the borate will ultimately be converted to an alkali borate of relative fixed type after heating to fusion and after a sufiicie'nt period of use wherein it is subjected to oxidation and reduction. Moreover, if the initial bath additive is hydrated borate salt, it becomes dehydrated since the fused bath will eflect a dehydration of the borate to a final dehy- It is preferred to use ordinary sodium starting material for the bath.

The bath may also be made up by mixing initial components of more varied character which, when interacted or fused in the bath to operative 01m, conforms to a composition as above described. For example, caustic soda may be mixed with alkali metal fluoborate, typically potassium or sodium fluoborate, which when fused into a bath forms a borate together with alkali metal fluoride. That procedure is not preferred since some water also is formed in the reaction which must be evolved by the bath. Such bath therefore cannot be formed by rapid heating, since the rapid evolution of water vapor such as steam from the bath may be dangerous. That reaction is illustrated in the following equation:

.Where the caustic is present in excess as usual according to the quantity stated above, then unreacted caustic soda will remain as the preponderant bath component.

As shown by this equation, the reaction of alkali metal fluoborate produces in the bath both borax and alkali metal fluorides. For production of the present bath composition the fiuoborate will be used in quantity sutficient to produce at least 5 percent mixture of borax and alkali metal fluoride by weight of the bath composition. The fluoborate may be either potassium fluoborate or sodium fluoborate.

Thus by starting with a major proportion of alkali caustic and a substantial but minor portion of the fluoborate such as 2 to 5 percent, the reaction provides sulficient borax in the bath with the major proportion-of the bath comprising that part of the alkali caustic which is not chemically reacted with the fluoborate. Thus, the

resultant bath contains a major part of sodium hydroxide or alkali caustic and minor parts of borax and alkali fluoride which reacts under electrolysis to produce the cleaning action described above.

In a typical electrolytic cleaning by this invention, the work piece to be cleaned, such as ferrous metal castings containing sand mold inclusion as Well as metal oxide scale, is first made a part of the cathode, dipped in the fused bath containing both borate and fluoride (since sand is to be removed with the oxide scale), subjected to 1,000 amperes of current for each 4,000 lbs. of bath substance and electrolyzed for about 15 minutes at a voltage drop of 2 /2 to 6 volts, at a temperature of about 850' F. Thereafter the current is reversed to make the work piece anodic for 15 minutes, and finally reversed again to be cathodic for 10 minutes. Upon removal from the bath and washing free ofmolten salt, the-metal is clean. 7

The bath composition will comprise from 75 to 95 percent of alkali metal hydroxide, typically caustic soda, 1 to 15 percent of an alkali metal borate compound, to 10 percent of an alkali metal fluoride, the combined quantities of alkali boron compound and fluoride being at least percent, and from 0 to 15 percent of sodium chloride.

The following examples illustrate bath compositions within the scope of this invention.

Example 1.-9 0 percent by weight of caustic soda, 10 percent by weight of borax.

Example 2. 85 percent by weight of sodium hydroxid'e, percent 'by weight of borax.

Example 3.85 percent by weight of sodium hydroxide, 10 "percent by weight of borax, and 5 percent by weight or sodium chloride.

Example 4.95 percent by weight of sodium hydroxide, 2 percent by weight of borax and 2 /2 percent by weight of sodium fluoride.

Example 5.85 percent by weight of sodium hydroxid'e, 5 percent by weight of borax, 2 percent by weight of sodium fluoride and 8 percent by weight of sodium chloride.

Them'olten'salt bath when ready for cleaning, may be operated at a temperature between 700 and 1000 F. However, the best operating temperature range is believed to be between 800 and 850 F.

It can be seen that the chemical composition of 'the bath may be arrived at by'first mixing the various chemicals as they are desired to appear in the molten salt bath, in the dry state, and then melting these chemicals, or in the alternative, the bath chemistry can be obtained by mixing compounds which react to form the chemical composition desired in the bath.

Iclaim:

1. Apr'oces'sof cleaning metal comprising fusing a mixture of alkali metal hydroxide in proportion of at least about -'75 percent by Weight with about 1 to 15 percent by weight of alkali metal borate, suspending the metal to be cleaned in said molten salt bath as anelectrode and passing an electric current therethrough.

{2. A processof cleaning metal comprising fusing a mixture of "alkali metal hydroxide in proportion of at least about 75 percent by weight with a relativelyminor quantity, at least one percent by weight of alkali metal borate and a minor quantity, at least one percent by weight alkali metal fluoride, suspending the metal to be cleaned in "said molten salt bath as an electrode and passing electric current therethrough.

-3.--.A p'rocess of cleaning metal comprising fusing a mixture of salts to form a molten salt bath, said'molten salt bath comprising at least one percent by weight of alkali metal borate, atleast one percent by weight of alkali metal fluoride and the remainder, comprising at least'about 75% by weight of caustic soda, suspending the metal 'to be cleanedin said fused bath as an electrode andpassing electric current 'therethrough.

4. The process of cleaning-metalcomprising fusing a mixture to form a molten salt'bath comprising at least about '95 percent by Weight of sodium hydroxide and the remainder a mixture totalling about 2 to 5 percent by Weight of both alkali metal borate and alkali metal fluoride, suspending the metal to be cleaned in said molten salt bath as an electrode and passing an electric current therethrough.

5. A molten salt bath composition for cleaning o'fmetal by electrolysis therein, comprising-atleast by weight of alkali metal hydroxide, at least 1% by weight of alkali metal borate and at least 1% by weight alkali metal fluoride.

6. A molten salt bath oompositionfor cleaning of metal by electrolysis therein, comprising 1 to 15% by weight of alkali metal borate, 1 to 15% by weight alkali metal fluoride and the remainder, comprising at least 75 by weight alkali metal hydroxide.

7. A molten salt bath composition for cleaning of metal by electrolysis therein, composed in its molten state of about 95% by weight of alkali metal hydroxide and a total of about 5% by weight, both in minor quantities, :of at least 1% by weight each of alkali metal fluoride and alkali metal borate.

8. A molten salt bath composition for cleaning of metal by electrolysis therein, consisting of about to by weight of alkali metal hydroxide and about 5 to 15% by weight of alkali metal borate.

9. A molten salt bath composition for cleaning of metal by electrolysis therein, comprising about 5 to 15% by weight of alkali metal borate, about 1 to 15% by weight of sodium chloride and the remainder, comprising at least "75% by weight alkali metal hydroxide.

10. A molten :salt bath composition for cleaning of metal by electrolysis therein, comprising about 2 to 5% by weight of an alkali metal boron and fluoride composition which in the molten form comprises both alkali metal borate and alkali metal fluoride salts each present in quantity of at least 1% by weight, and the remainder, comprising at least 75% by weight sodium hydroxide.

11. A molten salt bath composition for cleaning of metal by electrolysis therein, comprising at least about 95% by weight sodium hydroxide and about 2 to 5% by weight of an alkali metal boron and alkali metal fluoride composition which in the molten form comprises both alkali metal borate and alkali metal fluoride salts, each present in quantity of at least 1% by weight.

12. A molten salt bath composition for cleaning of metal by electrolysis therein, comprising about 1 to 15% by weight of alkali metal borate, 1 to 10% by weight of alkalimeta'l fluoride, about 1 to 15% by weight of sodium chloride and the remainder, at least 75% by weight, substantially sodium hydroxide. I

13. A .molten salt bath composition for cleaning of metal by electrolysis therein, comprising 75% to 95% by weight sodium hydroxide, 5 .to 15% by weight of borax and 1'to'15 by weight of sodium chloride.

'14. A .molten salt bath composition for cleaning of metal byelectrolysis therein, comprising about 85% by weight of sodium hydroxide, 8% by weight of sodium chloride, 2% by weight of sodium fluoride and 5% by Weight of borax.

15. A molten salt bath composition for cleaning of metal by electrolysis therein, comprising about 95% by weight of sodium hydroxide, about'2 /2'% by weight of borax and about 2 /2 by weight of sodium fluoride.

16. A molten salt bath composition for cleaning of metal by electrolysis therein, comprising about.85% by weight of sodium hydroxide, 10% .by weight of borax and 5% by weight of sodium chloride.

References Cited-in the file of this'patent UNITED STATES PATENTS 195,267 Elliott et'ral '.Sept. 18,1877 2,463,006 Webster r Apr. 19, 1949 'Clingan -e Oct. 13, 1953 

1. A PROCESS OF CLEANING METAL COMPRISING FUSING A MIXTURE OF ALKALI METAL HYDROXIDE IN PROPORTION OF AT LEAST ABOUT 75 PERCENT BY WEIGHT WITH ABOUT 1 TO 15 PERCENT BY WEIGHT OF ALKALI METAL BORATE, SUSPEDING THE METAL TO BE CLEANED IN SAID MOLTEN SALT BATH AS AN ELECTRODE AND PASSING AN ELECTRIC CURRENT THERTHROUGH. 